The invention generally relates to a soldering foil for the stress-free joining of ceramic bodies to metal, comprising two layers of an active solder provided with an intermediate cushion layer which absorbs stress. A process for manufacturing the subject foil is also disclosed.
To remove thermal stresses in solder layers, it is already known to use soldering foils which contain a cushion layer, for absorbing stress, between two solder layers (German Patent Application K 18,296/49h, (26/01), or Wiss. Z.d. Techn. Hochsch. Karl-Marx-Stadt, 10 (1968), No. 1, 83-89). Furthermore, a metal-ceramic component is known in which a layer of porous metal is inserted between two layers of active solder for absorbing stress (German Pat. No. 3,014,645).
However, the disadvantage in these joints or soldering foils is that, either the porous metal cushion layer is undesirably thick or thermal stresses are not reliably removed so that in some cases the soldering joints are weak. In addition, porous metal only permit small forces to be transmitted and there is also a danger that cavities existing in the porous metal will become filled with solder causing the entire compound structure to become brittle. When metal foils are used as the cushion layer, the soldering joints are extremely sensitive to fluctuations in the soldering process. Even slight fluctuations in the soldering temperature or the duration of soldering have a relatively large influence on the subsequent stability of the soldering joints.
Therefore, it is one object of the present invention to develop a soldering foil for joining ceramic to metal which leads to more solid soldering joints than hitherto obtainable.
Another object of the present invention is to develop a soldering foil which, during soldering, has a greater tolerance for deviations in soldering temperatures and soldering times than hitherto known soldering foils.
These and other objects are achieved by the multi-layer soldering foils described herein.
The subject invention relates to a soldering foil having a multi-layer structure. The solder used in this arrangement is one which forms a chemical bond with ceramic at the soldering temperature (a so-called active solder). In this manner a particularly intimate connection between solder and ceramic material is achieved. In the case of oxide, nitride or carbide ceramics, titanium-containing active solders are normally used because the high affinity of titanium results in breakage of the oxide, nitride or carbide bonds of the ceramic surface, thereby producing a titanium oxide, nitride or carbide transition region with high adhesive strength. Titanium-containing active solders are known, for example, with a silver, silver/copper, silver/zirconium and copper base. Active solders with a copper base are particularly inexpensive. In these solders, the titanium content is between 20 and 43% by weight. A solder with a copper base with 22 to 28% by weight of titanium is particularly preferred. The active solder layer will typically be between 30 and 80 .mu.m thick.
A cushion layer for absorbing stresses is placed between the two active solder outer layers of the soldering foil. This cushion layer can consist of, for example, copper, iron, nickel, iron-nickel alloy, copper-beryllium alloy or low-carbon steel. Because of its high ductility, a copper foil is preferred in this arrangement. The thickness of the copper foil is normally matched to the magnitude of the natural stress to be compensated. The cushion layer usually has a thickness of from 50 to 300 .mu.m.
A barrier layer of silver is arranged between the cushion layer which compensates for stresses and each active solder layer. This barrier layer is essential for the durability of the soldering joints and desirable behaviour of the soldering foil during the soldering process. The silver layer can be extremely thin with a thickness normally between 2 and 10 .mu.m. The barrier layer can be produced by various processes, such as, for example, by electrolytic coating of the copper foil or of the active solder layer, by inserting a silver foil between the active solder layer and the cushion layer or, also by sprinkling a layer of silver powder onto the cushion layer before joining the cushion layer to the active solder layer.
Further objects, advantages and novel features of the present invention will become more apparent from the following detailed description when considered in conjunction with the accompanying drawings which show, for purpose of illustration only, embodiments in accordance with the present invention.